It is well known in the medical electrode art that an ECG electrode must make good electrical contact with a patient in order for the electrode to accurately sense the patient's intrinsic ECG rhythm. External ECG electrodes are usually affixed to a patient's skin by the adhesive properties of the electrode, such as by the surface tackiness of an electrolytic gel forming a part of the electrode. When a good electrode-to-skin connection is made, the impedance of the connection is low. If the connection is not good, the connection impedance will be higher than the impedance of a good electrode-to-skin connection. A high ECG electrode-to-skin connection impedance will hinder the electrode's ability to sense the patient's intrinsic ECG rhythm. A sufficiently high electrode-to-skin connection impedance will result in a loss of patient ECG signals to the ECG electrode. In such a situation, ECG monitoring equipment may detect this loss of ECG signals as a loss of cardiac activity in the patient, and alert medical personnel accordingly.
In applications where the ECG signals sensed by the ECG electrodes are used to control other types of equipment, the other equipment may operate improperly due to a loss of the ECG signal. For example, demand mode pacers are designed to apply external pacing pulses to a patient upon demand, i.e., when there is an absence of intrinsic ECG rhythm. If, due to a high ECG electrode-to-skin connection impedance, the ECG signals are blocked from the demand mode pacer, the pacer may sense this as a loss of patient ECG and begin applying pacing pulses to the patient. Obviously, unnecessarily applying pacing pulses to a patient with a normal ECG rhythm may be detrimental to the patient's health.
One method commonly used in the prior art to sense the integrity of the ECG electrode-to-skin connection is to continuously sense an overall lead impedance associated with a particular electrode. The lead impedance associated with an electrode actually comprises several impedances, including an impedance of the electrode conductor, an impedance of the electrode, an electrode-to-skin connection impedance, an internal impedance of the patient, and various other impedances, including those formed by connections of the conductor to the electrode and to other pieces of equipment. Typically, the impedance of the electrode-to-skin connection is the single largest component of the lead impedance associated with a particular electrode. This is especially true where the electrode becomes detached from the patient. Generallly, the prior art senses lead impedance in one of two ways, either by applying a DC signal to the conductors, or by applying a high-frequency AC signal to the conductors. In both prior art methods, the DC or AC signal produces a voltage that is proportional to the lead impedance associated with the electrode. This voltage(s) can then be processed in several ways, but, basically, it is compared to a threshold level, and if it equals or exceeds the threshold level, a poor electrode-to-skin connection (or leads-off condition) is presumed.
One problem associated with the prior art is that the prior art devices sense the lead impedance associated with a particular electrode without regard to the lead impedance associated with other electrodes. As a result, the prior art does not take into account variations between patients or electrode preparation techniques. For example, if a patient has dry skin, the impedance of the electrode-to-skin connections for any electrode attached to that patient will cause the lead impedances associated with those electrodes to be higher than if the electrodes were connected to a patient wth moister skin. Likewise, if a certain type (or brand) of electrode is used on a patient, the impedance of those types of electrodes may cause the lead impedance associated with those electrodes to be different (i.e., higher or lower) than for another type of electrode. Since the prior art senses the lead impedance associated with a particular electrode without regard to other electrodes, the high lead impedance caused, for example, by the dryness of the patient's skin, or by the type of electrode used, may be sensed as a leads-off condition. This may occur even though adequate electrical signals, such as ECG signals, are being received by ECG monitoring equipment attached to the ECG electrodes.
Another problem associated with the prior art concerns the signals used to sense the lead impedance. Some forms of the prior art use DC signals to sense the impedance. Patient movement may cause low-frequency modulation of the DC signals. These low-frequency modulations (in the neighborhood of 1 Hz) may corrupt other electrical signals present, such as the ECG signal. As a result, the ECG monitoring equipment may interpret the corrupted ECG signal as an abnormal patient ECG signal. A problem with the high-frequency AC signals used in the prior art is that, at high frequencies, the capacitance of conductors, such as the ECG conductors, that connect the ECG electrodes to the monitoring equipment becomes significant, and may cause erroneous lead impedance values to be sensed.
As can be readily appreciated from the foregoing discussion, there is a need for a method and apparatus for sensing the integrity of electrode connections generally, and more particularly, the integrity of ECG electrode connections that will take into account, among other factors, patient-to-patient and electrode variability. Such a method and apparatus should discriminate between an electrode leads-off condition and a situation where a high lead impedance is caused by a patient's skin or by a particular type of electrode, for example. Furthermore, such a method and apparatus should not corrupt other electrical signals present, such as patient ECG signals. The present invention is directed to providing such a method and apparatus.